1. DYNAMICS
IN THE
ver 15,000 years
ago, at the
t
most
POSTGLACIAL ERA
peak of the
of
present-day
glaciation,
was covered by ice up
to a mile thick. Cape Cod and the offshore
islands-Nantucket, Martha’s Vineyard, and the
Elizabeth Islands-consist largely of what geologists call moraines, piles of debris accumulated
at the front of the advancing ice sheet and left
behind when the glaciers finally melted. The
advancing glaciers not only smoothed and
shaped the landscape by scraping and plucking
the bedrock as they advanced, they also left
behind a layer of ground-up rock, or till, which
has since evolved into soil. As the glaciers
melted, the tremendous volume of water produced seasonal streams that carried and sorted
much of this material and deposited sands and
gravels wherever they slowed.’ The soils of
Massachusetts are a product of this massive
natural engineering, later supplemented by
orgamc material from the vegetation that eventually covered the landscape. Along major rivers
fine silt was deposited when the rivers overflowed their banks in spring; in some depressions a surplus of moisture allowed thick layers
of peat or muck to develop. The resulting pattern of soil types has strongly influenced the
type and distribution of trees and forests.
Of course, during the glacial period, there
were no forests in Massachusetts: The modern
tree species of New England were restricted to
favorable locations called refugia south of the
glacial zone, presumably scattered across the
southern Appalac ~ians and the eastern coastal
plain. The huge quantities of water trapped on
land as glacial ice had once been seawater; consequently, sea level was several hundred feet
lower than at present, leaving vast areas of continental shelf off the present-day East Coast
exposed as refugia as well. The forests in these
various refugia contained species mixtures
last
Massachusetts
unlike any we are familiar with today. As the
climate warmed and the glaciers melted, the
trees began their migration north at rates determined by the method of seed dispersal and the
climatic tolerance of each species.
We are fortunate to have a record of this postglacial migration in the form of pollen preserved
in sediments at the bottom of lakes and ponds
and below many wetlands. Each year pollen
from plants is carried away on the wmd. When
it lands on the water of a pond or lake, it sinks
to the bottom, and along with other wind-borne
material is incorporated into the sediments.
These sediments form a chronological sequence
of layers, the oldest at the bottom, which scientists can recover as a thin cylinder or core of
mud. Each layer in the core can be dated using
radioisotopes, and since pollen is extremely
resistant to decay-the pollen of different plants
can be identified at least to genus and in some
cases to species-the presence and relative
abundance of different types of pollen at each
layer enables paleoecologists to reconstruct the
major vegetation changes
at a site.
The pollen diagram for the Harvard Forest
Black Gum Swamp (page 6) shows changes in
vegetation over time that are typical of sites
across Massachusetts. As the glaciers melted
and the climate warmed, a period of tundra is
followed by boreal (northern conifer) forest,
succeeded in turn by pine forest with rapidly
increasing amounts of several deciduous species
(birch, oak, beech) by 8,000 years ago. Although
mixed deciduous forests have been dommant for
the last 8,000 years, the mixture of species has
changed continually during that period, and
these changes can tell us much. Species have
behaved quite independently, presumably
migrating to Massachusetts from different locations at different rates, each species responding
in its unique fashion to the various combina-
5
Alders, bearberry, and other pioneer vegetation follow a receding glacier m Alaska.
climatic, soil, biotic, and historical facfound in the area. The major influences on
these changes are long-term climate change,
migration rates of individual species, and natural disturbance processes.
Population dynamics of selected species
help us understand these processes. Hemlock
increased rapidly m importance after its arrival
about 9,000 years ago. A little less than 5,000
years ago, it decreased dramatically in a very
short time, then slowly recovered. This sudden
decline, seen in pollen records throughout the
Northeast, has been attributed to a severe outbreak of insects or disease that greatly reduced
hemlock populations for nearly 1,000 years.
Regional pollen analyses suggest that temperatures during the period from 8,000 to 5,000
years ago were milder than those of the last
4,000 years.2 It was during this warm period that
many tree species now common in Massachusetts migrated into the area, with some species
tions of
tors
extending well north of their current ranges. At
a number of sites, fire frequency, indicated by
charcoal in sediment cores, was also greater
than in the more recent period.3 The increase
in spruce pollen over the last 2,000 to 3,000
years was probably caused by more recent cooling. All these changes in forest communities
were complex: while spruce, a northern species,
increased-emdently in response to gradual
cooling-chestnut, a southern species, was also
migrating north across Massachusetts. In fact,
chestnut is the most recent arrival in the pollen
record, appearing less than 3,000 years ago,
much later than the other important deciduous
species that occur in the region today.
The Natural Environment
The distribution of forest types across Massachusetts during the period preceding European
settlement was controlled by the physical geography, or physiography, of the landscape; the
diagram from the Black Gum Swamp at the Harvard Forest m central Massachusetts depicting the
changes m the vegetation over the past 12,000 years Tundra communities were replaced by boreal forest
dominated by spruce until approximately 9,200 years before present (B P.) when pine and other tree species
Pollen
ma~or
became important.
Changes m the relative abundance of species resulted from chmate change, speczes migrations, disease
(hemlock declme at about 5,000 B Pand fire until 250 to 300 years B P, when European settlement resulted
m major deforestation and the mcrease m agricultural weeds, herbs, and early successional species
.4
underlying geology; and the frequency of distur-
Taconic Mountains. The
bances such
ies across
as
windstorm and fire. Massa-
chusetts, excluding Cape Cod, is roughly
rectangular, 125 miles (200 km) east to west
and 50 miles (80 km) north to south. Today it
receives approximately 40 mches (100 cm) of
precipitation annually, distributed fairly evenly
throughout the year, with a mean annual temperature that ranges from a mean of several
degrees Fahrenheit below freezing in January to
about 70 degrees in July, and averages nearly 50
degrees over the year.
Within a relatively small, compact area, Massachusetts contains six broad physiographic
regions: the coastal lowlands, the central
uplands, the Connecticut River valley, the Berkshire Mountains, the Berkshire valley, and the
the
state.
geologic substrate var-
Except for parts of the Con-
valley, the Taconic Mountains,
and the Berkshire valley, the soil is acidic, poor
in nutrients, and shallow with patches of
exposed bedrock. Elevation generally increases
from east to west, reaching a maximum at Mt.
Greylock in the Berkshires (3,487 ft [1060 m]).
These are the physiographic and geological conditions that interact with climate to produce
necticut River
vegetation
ecoregions.
zones
sometimes referred to
as
Within Massachusetts natural vegetation
largely determined by climate, which
in general varies with elevation except in areas
near the moderating influence of the ocean.
Southeastern Massachusetts, all of Cape Cod,
zones are
7
forests of the central Appalachians and the Middle Atlantic States.
North and west of the central hardwood zone
we generally find the transition hardwood zone,
which also extends up the major river valleys in
the western part of the state. This zone is characterized by increasing amounts of species
found farther north, such as yellow birch, black
(or sweet) birch, sugar maple, and beech, with
less oak (especially white oak) and with paper
birch occurring on heavily disturbed sites. The
higher elevations in the Berkshire and Taconic
mountains and the extreme northern border of
central Massachusetts fall within the northern
hardwood and spruce-fir zones. The spruce-fir
zone, m which red spruce is the dominant conifer, is restricted to the highest elevations, generally above 2,000 feet, while the northern
hardwood zone occurs just below the spruce-fir
zone and has hemlock and white pine as its
dominant conifers. Both zones have mixtures of
hardwoods dominated by sugar maple, yellow
birch, beech, and red maple.
hickory dominated
Map of southern
England depicting the maior
From east to west they are
coastal lowlands, central uplands, Connecticut
River valley, Berkshire Mountams, Berkshire valley,
New
physiographic regions.
and Tacomc Mountams.sb6
and the offshore islands fall
within the pitch pine-oak zone.
This vegetation type, occurring
on sandy and gravelly soils laid
down as glacial moraines or outwash deposits, is characterized
by drought-tolerant and fire-
adapted species, including pitch
pine, scrub oak, and huckleThis type also occurs
scattered outwash deposits
in inland Massachusetts. The
remainder of the coastal lowlands, the southern central
uplands, and the southern
Connecticut River valley fall
withm the central hardwoodhemlock-white pme zone. This
vegetation type represents the
northern extension of the oak-
berry.
on
The ma~or forest vegetation zones m southern New England. Sandy
glacial deposits, found on Cape Cod and m extreme southeastern
Massachusetts, support a dry forest of pitch pine and scrub oak
Transition hardwood forest dominates the central uplands and much of
the Connecticut River and Berkshire valleys, with northern hardwood
forest on the higher elevauons m the Berkshires and Tacomcs and sprucefir forest on the highest e~evatjons ~I